1. Field of the Invention
The present invention relates generally to inkjet recording apparatuses, and more particularly, to an inkjet recording apparatus for recording images by expelling ink droplets using a piezoelectric element.
2. Description of the Related Art
Among conventional inkjet printers, some known printers use a piezoelectric element (PZT) for the head. In such a head, a pulse voltage corresponding to image information is applied to the piezoelectric element, and distortion caused by the applied pulse voltage pressurizes ink in a prescribed container (ink channel), which causes ink droplets to be expelled form the nozzle provided at the ink channel toward a recording sheet. A rectangular pulse is conventionally known as the kind of pulse voltage applied to such a piezoelectric element.
FIG. 19 is a waveform chart showing rectangular pulse voltages as conventionally used. In these pulse voltages, the rising time, duration of the pulse amplitude, falling time, size of the pulse amplitude or the like is adjusted to control the speed, size or the like of droplets to be expelled.
The use of such a rectangular pulse voltage however may cause ripples in the pressure applied to ink caused by the vibration of the piezoelectric element for the duration of the pulse amplitude (voltage hold period).
FIG. 20 is an illustration of a narrowed part of an expelled ink produced by the vibration of the piezoelectric element during the voltage hold period. FIG. 21 is an illustration of ink droplets produced by the expelled ink having a narrowed part.
As shown in FIG. 20, an ink pillar 200 having narrow parts 201, 202 and 203 is produced from a nozzle 150 by applying a pulse voltage having a voltage hold period as shown in FIG. 19. An ink droplet 251 with satellites 252 and 253 is produced as shown in FIG. 21. Images formed on a sheet by such droplets are considerably degraded because satellites 252 and 253 stick at unexpected positions or the dot size becomes unstable by change in the volume of ink droplet 251.